Modern high technology manufacturing processes often use highly purified "ultrapure" water in large quantities. The semiconductor industry in particular uses ultrapure water as a universal solvent in virtually every step of the production of integrated circuits. In recent years, it has been recognized that even trace amounts of organic contamination in the water, though often undetectable by the commonly-used ionic (i.e. conductivity-based) measurement techniques, can severely degrade both product quality and yield. Accurate and continuous monitoring of the total organic content is crucial if these problems are to be avoided. Similar problems exist through other industries, such as pharmaceutical and chemical manufacturing.
Several approaches to measurement of the organic content of water have been proposed. Those relevant to the present invention are primarily concerned with oxidation of the carbon in the organic material to carbon dioxide and then measuring the carbon dioxide content. This has been done in the past in several ways. The fact that carbon dioxide is an absorber of infrared light has been utilized. The oxidation has also been performed in several ways, including combustion, and using chemical oxidizers such as perchlorates. These methods are clumsy and are replete with the potential for significant errors, particularly in the low-TOC area addressed by the present invention. More relevant to the present invention is the approach shown in U.S. Pat. No. 3,958,941 to Regan, in which ultraviolet light is used to oxidize the carbon-containing organics in a water sample to carbon dioxide. (As a practical matter, in use of the Regan apparatus, the pH of the water sample is adjusted if necessary to ensure that the CO.sub.2 is gaseous.) The carbon dioxide is then transported to a pure water sample, in which the CO.sub.2 is ionized, as H.sup.+ +CO.sub.3.sup.- - ions. The change in conductivity of the pure water due to the presence of the additional ionic species is monitored to determine the amount of organic material thus oxidized. Oxidation of the organics to CO.sub.2 and measurement of the change in the water's conductivity are used by the apparatus of the present invention. However, several improvements over the Regan apparatus are shown herein.
The Regan apparatus, which is commercially available, is proposed as a tool for measuring organic content of water over a wide range, from the parts per million (ppm) range through parts per thousand and, indeed, even higher. Applicants have had experience with this apparatus, however, and find that the problems inherent in total organic carbon measurement at extremely low dissolved organic levels, on the order of one part per billion (ppb) to one ppm are such that a different type of apparatus should be used for these extremely low level measurements. Thus, while the Regan approach is workable, it is of primary utility in the areas of relatively high organic concentrations.
The Regan apparatus requires the operator to perform several independent preliminary measurement runs to determine the "instrument contribution" or background level of the instrument. The inventors have found that the values determined in such measurements tend to change with time, thereby requiring frequent "calibration" runs to maintain measurement accuracy.
It is therefore an object of the invention to provide a means whereby the instrument contribution can be accurately determined separately from the measured quantity, such that calibration runs are eliminated.
The Regan apparatus assumes a fixed time for the oxidation process to go to completion. If the organics present in the sample are difficult to oxidize, or if the ultraviolet lamp has aged so as to produce insufficient oxidizing radiation, they may not be completely oxidized in the time allotted, thus leading to misleadingly low measurements. Furthermore, if the level of organics is very low and oxidation proceeds to completion rapidly, the interference caused by instrument contribution may contribute significant errors.
It is therefore a further object of the invention to provide an instrument whereby the oxidation process can be monitored so that its actual completion can be accurately and readily determined.
As mentioned, the Regan apparatus provides a two-loop system, in which the organics in water are first oxidized by exposure to ultraviolet (UV) light, and the resulting carbon dioxide transferred to a measurement chamber, where it is dissolved in pure water, the conductivity of which is thereafter measured. The conductivity is thus measured in a different chamber than that in which the ultraviolet light is exposed to the water. This has the highly significant defect that transport of the carbon dioxide between the UV exposure chamber to the conductivity measurement chamber is obviously required. The present invention is designed to address measurement of the organic content of water in such low concentrations that any minor impurities which are added to the water by this or any comparable transport system (as well as loss of CO.sub.2) can very greatly affect the accuracy of any measurement.
Accordingly, it is an object of the invention to provide an instrument for the measurement of total organic carbon in water which avoids water, CO.sub.2 or other material handling or manipulative steps, such that the impurities inevitably added in such steps are avoided.
The present invention overcomes the problems associated with the defects of the Regan apparatus due to its transport and manipulative step requirements by providing a single chamber in which the ultraviolet radiation is exposed to the water and in which the conductivity measurements are made. This has several advantages, among which are, of course, reduction of pollutants or contamination due to transport, simplicity, and low cost. Furthermore, the fact that the electrodes can be and are in a preferred environment exposed directly to the UV light means that there is no or very little chance of organic fouling of the electrodes, another problem inherent in the Regan apparatus according to the two-chamber approach proposed thereby.
It is accordingly an object of the invention to provide an instrument for measurement of total organic carbon in water in which a static water sample is measured for background conductivity, is then exposed to ultraviolet light, and variation in its conductivity is measured over time, without movement from a single sample chamber, whereby inaccuracies due to manipulative steps are eliminated.
It is a further object of the invention to provide such an organic matter measurement instrument in which the electrodes used for conductivity measurement are directly exposed to the ultraviolet light used to oxidize the organic carbon to carbon dioxide, such that organic fouling of the electrodes is avoided.
It is a further object of the invention, in accordance with good design practice, to avoid use of materials in contact with the water sample which could lead to leaching of additional impurities, such as iron, polyethylene, and other materials found in prior designs, and instead to permit only relatively inert materials such as titanium or quartz to come into contact with the water sample.
As mentioned above, according to the invention, it is desired that a static water sample be examined; that is, according to the invention, a water sample is taken from the process of interest. The testing according to the invention is thus not an in-line process, as that term is typically used, although, in fact, the time taken for a typical measurement, on the order of one to twenty minutes, is such that substantially up-to-date information can be provided. (Note however that in some circumstances, detailed below, the apparatus of the invention can be used to monitor sudden changes in total organic carbon content in an in-line, real-time manner.) The art generally teaches away from such static measurements, because it is known that the materials comprising the electrodes used for the resistive measurements as well as those of the sample chamber tend to be leached out into the water stream and make some contribution to the conductivity of the water. The more delicate the measurement, the more serious these contributions can be. Use of a flowing water stream has been suggested to minimize the effects of such additional ions which alter the conductivity.
It is a further object of the invention to provide a means by which the instrument contribution or "background" conductivity can be accurately determined and subtracted from the total measured value for conductivity, thus permitting use of a static sample measurement technique.
According to the present invention, accurate compensation is made for the instrument contribution due, e.g., to its materials leaching over time, so that the other advantages of static measurement can be realized, and so that the instrument contribution to conductivity, regardless of its source, is prevented from interfering with accurate measurement.
As mentioned, according to the process of the Regan patent, the conductivity of the water in a measurement chamber is first measured. The water sample of interest is exposed to ultraviolet light in a second exposure chamber for a fixed length of time. The carbon dioxide is then removed and dissolved into the water in the measurement chamber. The conductivity of the water is then measured and compared to its conductivity at the beginning of the exposure period. The difference is taken to be indicative of the change in conductivity due to CO.sub.2 formed by oxidation of organic carbon. Because the relationship of conductivity of water to carbon dioxide content is known, this can be used to directly derive a measurement of organic carbon content. There are several difficulties inherent in this approach. One is that the background noise or instrument contribution, including the additional conductivity caused by leaching of organic or inorganic materials of the apparatus, is not repeatable over time, a fact brought out by the inventors' experiments. Furthermore, the dependence of conductivity of water on carbon dioxide content is not a linear function, but is exponential, such that at higher organic carbon contents, relatively little conductivity change is experienced with significant variation in organic carbon content. Hence, accurate determination of the background level is essential if an accurate measurement of organic content is to be provided.
Accordingly, it is an object of the invention to provide a method and instrument for measurement of the organic content of water in which accurate background compensation is made, yet in which background compensation is not dependent on repeatability of background measurement, and wherein compensation is made for any chemical activity of the sample chamber or apparatus caused by ultraviolet light, and wherein the compensation for background is sufficiently delicate that the precision of result necessary for distinguishing between conductivity caused by various relatively low amounts of organic content is made possible.
One primary difficulty with prior art TOC measuring instruments is that all presently available devices require frequent and tedious calibration, due largely to the high and somewhat varying instrument contribution or background.
Accordingly, it is an object of the invention to provide a TOC measuring instrument, the absolute calibration of which is made solely by correctly calibrating an integral temperature-corrected conductivity sensor.
It is a further object of the invention to provide a TOC measuring instrument which automatically detects and compensates for such spurious background, substantially eliminating the need for frequent calibration.